Citation for published version:Alafnan, H, Elshiekh, M, Pei, X, Altouq, S, Fazeli, S, Sun, Q, Zhang, M & Yuan, W 2019, 'Application of SMES-FCL in Electric Aircraft for Stability Improvement', IEEE Transactions on Applied Superconductivity, vol. 29, no.5, 8668791. https://doi.org/10.1109/TASC.2019.2905950

DOI:10.1109/TASC.2019.2905950

Publication date:2019

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https://doi.org/10.1109/TASC.2019.2905950https://doi.org/10.1109/TASC.2019.2905950https://researchportal.bath.ac.uk/en/publications/application-of-smesfcl-in-electric-aircraft-for-stability-improvement(f3b652de-d235-4722-8c37-1d651d6fc6a9).html

Application of SMES-FCL in Electric Aircraft for

Stability Improvement Hamoud Alafnan, Mariam Elshiekh, Xiaoze Pei, Shadan Altouq, Seyed Mahdi Fazeli, Qixing Sun,

Min Zhang, Weijia Yuan

Abstract— The increase in aircraft passengers and airfreight

traffic has given rise to concerns about greenhouse gas emissions

for traditional aircraft and the resulting damage to the environ-

ment. This has led several companies and organizations, including

NASA, to set goals to enhance aircraft efficiency as well as reduce

fuel burn, pollution, and noise for commercial aircraft. The most

notable electric aircraft (EA) concept is the N3-X, which was de-

veloped by NASA to achieve environmental goals while maintain-

ing the annual growth of the aviation industry. However, one of

the main challenges that EA facing is their overall weight. This pa-

per proposes and explores an improved power system architecture

for use in EA based on the N3-X concept. The number of super-

conducting magnetic energy storage (SMES) and fault current

limiter (FCL) devices required can be reduced by utilizing multi-

functional superconducting devices that combine the functionali-

ties of both a SMES and a FCL, thus reducing the weight and cost

of the EA by eliminating a complete device. The proposed control

technique offers greater flexibility in determining the appropriate

size of coils to function as a FCL, based on the fault type. The pro-

posed EA power system architecture including the SMES-FCL de-

vices is modelled in Simulink/Matlab to test the system perfor-

mance under different failure scenarios.

Index Terms——Electric aircraft (EA), fault current limiter

(FCL), superconducting magnetic energy storage (SMES), turbo-

electric distributed propulsion system (TeDP).

I. INTRODUCTION

RANSPORTATION and electricity generation are the largest

sources of carbon dioxide (CO2) emissions in the US, at

34% per each, 68% in total [1]. The annual increase in aircraft

passengers is estimated to be 6.5%, while the annual freight

traffic growth rate is 4.4% [2], meaning that with current avia-

tion transportation technology, CO2 emissions will continue to

increase dramatically. Because concerns about global warming

and pollution are increasing, many companies and organiza-

tions have set goals to limit atmospheric pollution and

This work was funded as part of the UK EPSRC, Developing Superconducting Fault Current Limiters (SFCLs) for Distributed Electric Propulsion Aircraft: EP/S000720/1.

H. Alafnan, M. Elshiekh, X. Pei, Sh. Altouq, S. Fazeli and Q. Sun are with the Department of Electronic and Electrical Engineering, University of Bath, Bath BA2 7AY, UK (e-mail: hfa25@bath.ac.uk). M. Elshiekh is currently visiting the Department of Electronic and Electrical Engineering, University of Strathclyde

M. Zhang, W. Yuan are with the Department of Electronic and Electrical Engi-neering, University of Strathclyde, Glasgow, G1 1XW, UK.

(e-mail: Weijia.yuan@strath.ac.uk) H. Alafnan is with the Department of Electrical Engineering, University of Hail,

Hail, 55476, KSA. M. Elshiekh is with the Department of Electrical Power and Machines Engi-

neering, Faculty of Engineering, Tanta University, Tanta 3152, Egypt.

TABLE I

NASA AND ACARE ENVIRONMENTAL GOALS

Category ACARE

2020 ACARE

2050 NASA N+2

~2020 NASA N+3

~2030

CO2 reduction 50% 75% - -

NOx reduction 80% 90% 75% 80%

Ex. noise 50% 65% -42 dB -71dB Fuel burn 50% - 50% 60%

greenhouse gas emissions, such as the National Aeronautics and

Space Administration (NASA) and the Advisory Council for

Aviation Research and Innovation in Europe (ACARE). The

NASA and ACARE environmental goals relative to year 2000

are shown in Table I [3], [4]. As can been seen in Table I, the

targeted improvements for both NASA N+3 and ACARE 2050

are extremely high, whereby reductions of CO2 by 75%, NOx

by 90%, and external noise by 65% are infeasible in traditional

aircraft design (gas turbine or piston engine) due of the rela-

tively low efficiency ~ 40% [5]. To achieve such goals, the air-

craft, including the propulsion system, must work with superior

efficiency.

The most notable EA concept is the N3-X, which has a

range of 22.5 MVA for passenger aircraft and was proposed by

NASA’s Research and Technology for Aerospace Propulsion

Systems (RTAPS) [6]. The N3-X combines the advantages of a

turboelectric distributed propulsion system (TeDP), boundary

layer ingestion (BLI), and superconducting technology to

achieve the highest possible efficiency with the minimal

weight. The N3-X power system architecture, known as the In-

ner Bus Tie Concept (IBTC), consists of four generators that

supply 16 propulsors throughout a DC microgrid to give the re-

quired thrust.

However, there is a possibility that short fault currents may

occur when this system is employed in larger passenger aircraft

due to significant in-flight vibration or adverse weather condi-

tions. Such occurrences may consequently cause serious perma-

nent faults that can, if not properly addressed, lead to onboard

fire, power disruption, system damage, or catastrophic failure

[7],[8].

In the N3-X concept, NASA proposed the use of supercon-

ducting fault current limiter (SFCL) and superconducting mag-

netic energy storage (SMES) devices. A SFCL provides very

effective current limitation within a few milliseconds [9]–[11].

which can offer a solution to the issue of short fault currents.

SMES devices have a higher power density, faster time re-

sponse, and unlimited charge and discharge lifecycles com-

pared to other energy storage technologies [12],[13]. Such ad-

vantages are important for EA performance, where system reli-

ability is a crucial point.

T

mailto:hfa25@bath.ac.ukmailto:Weijia.yuan@strath.ac.uk

Fig. 1: Improved power system architecture based on N3-X

This paper proposes the use of a multifunctional superconduct-

ing device that can be used as both a SMES and FCL in EA, by

using the same coils for the two modes, the SFCL devices with

their components are eliminated as shown in Fig. 1. The elimi-

nation of a complete device can reduce the overall weight and

cost of the EA. The proposed control algorithm allows the super

conducting coils to work as a SMES or part of the coils as a

FCL, based on the fault type. Multipurpose superconducting

coils have been proposed in several applications, including in

wind farms for AC and DC networks [14],[15]. For a DC net-

work, previous work has suggested the use of the whole coil as

a SMES or a FCL. However, the proposed control technique

offers greater flexibility to determine the appropriate size of the

coils to work as an FCL based on the fault current. The pro-

posed power system architecture, including the control algo-

rithm, is modelled and tested under different fault scenarios us-

ing the Simulink/Matlab environment.

II. SYSTEM DESCRIPTION

The N3-X TeDP power system architecture, the Inner Bus

Tie Concept (IBTC), proposed by NASA [16], was chosen as

the platform for testing the performance of the SMES-FCL de-

vices under various fault scenarios. The system components are

shown in Table II.

The capacities of the generators, motors, and converters are

based on the data of the aircraft proposed by NASA [16]. The

SMES-FCL device capacity is calculated to supply a set of four

motors for 320 ms at a cruise rated power of 1.5625 MW per

motor. The propulsion system is required to produce 22.5 MW

for maximum thrust during take-off [17].

TABLE II

THE DESIGN PARAMETERS OF THE EA

Parameter Quantity Value

Generator 2 2

14.91 MW, 6 kV (GR-2, GL-2) 7.46 MW, 6 kV (GL-1, GR-1)

Motor Converter, AC/DC

Converter, DC/AC

16 2 2

16

1.86 MW 14.91 MW 7.46 MW 1.86 MW

SMES-FCL 4 2 MJ (=0.556 kWh)

Because each motor can produce 1.86 MW thrust, at least 12

motors are required to work at the same time to ensure a stable

operation. The voltage DC-link is rated at 6 kVDC, as recom-

mend by NASA [18]. The multifunctional superconducting de-

vices can replace the SMES and SFCL devices, which helps to

reduce both the weight and cost of the EA while maintaining

the same performance. In this paper, half of the power system

architecture of the N3-X has been modelled, with eight motors

(1.86 MW) and two generators (14.91 MW) as shown in Fig. 1,

instated of the full sixteen motors and four generators [16].

A. SMES-FCL Device.

Due to its fast response, a SMES works well to maintain the

voltage at the DC-link and ensures a stable operation for the

propulsion system, which is a crucial for the EA design. SMES

devices have been proposed and used in several applications,

including a hybrid vehicle, electric ships, and microgrids [19]–

[21]. The stored energy of a SMES is calculated as follows:

𝐸𝑠𝑚𝑒𝑠 =1

2𝐿𝐼𝑆𝑀𝐸𝑆

2 (1)

where 𝐿 is the inductance in Henry, 𝐼𝑆𝑀𝐸𝑆 is the current stored in the SMES, and 𝐸𝑠𝑚𝑒𝑠 is the stored energy in SMES in Joule. Resistive type superconducting fault current limiters are

considered self-recovery devices. When the current passing

through the SFCL coils exceeds their critical current, the SFCL

resistance starts to increase dramatically, according to the fol-

lowing equation [22]:

𝜌𝐻𝑇𝑆 =𝐸𝑐

𝐽𝑐(𝑇)(

𝐽

𝐽𝑐(𝑇))𝑁−1 𝑇 < 𝑇𝑐 , 𝐽 > 𝐽𝑐 (2)

M

NC : Normally Close NO: Normally Open

Generator Motor

Fan or PropellerRectifier

Diode

DC/AC VFD SMES

Circuit Breaker

NC

NC

NC

SMES-FCL

SFCL

SFCL

NC

ML1

NC

NC

NC

NC

SMES-FCL

SFCL

SFCL

NC

SFCL

NO

Fault#1

GRGL

NC

NC

NC

Fault#2

MR1

ML4

MR4

ML2

MR2

ML3

MR3

where Ec = 1μV/cm is the critical electrical field. The N value

is usually between 21 and 30 for Yttrium Barium Copper Oxide

(YBCO) tapes. When the applied current is greater than the crit-

ical current, a joule heating effect occurs due to the exponential

rise in 𝜌𝐻𝑇𝑆, leading to a rise in the temperature of the super-conducting material.

𝐽𝑐(𝑇) = 𝐽𝑐𝑜 ((𝑇𝑐 − 𝑇(𝑡))

α

(𝑇𝑐 − (𝑇𝑜)α) 𝑇 < 𝑇𝑐 (3)

where 𝛼 is 1.5, which is applicable to YBCO superconducting materials, 𝐽𝑐𝑜 is the critical current density at the initial temper-ature 𝑇𝑜. As the current density 𝐽𝑐(𝑇) is less than the critical current density 𝐽𝑐𝑜, the coils’ resistance will be neglected. How-ever, when the current passing through the coils exceeds the

critical current, the coils resistance starts to increase and limits

the high fault currents. The concept of the resistive type SFCL

is used to limit the fault currents in the EA system using the

SMES coil.

The SMES-FCL device provides the two types of operation

by using the same coils. For the SMES operation mode, the

whole coil can be used to achieve the highest energy capacity.

However, a few pancakes will be enough to achieve the desired

limitation without affecting the system stability or the coil it-

self. In this study, a resistance value of 2 Ω is used as the fault

current limiting resistance.

The SMES coils comprise 67 pancakes with an inductance

of 1.005 H and a current rating of 2 kA. The SMES capacity is

calculated by (1) to be 2 MJ [23],[24]. With regards to YBCO

material containing copper stabilizer, it is possible to achieve a

resistance value of 2 Ω through the use of two pancakes,

whereby each one consists of 50 m of superconducting tape. If

it were wound into a single pancake structure with an inner di-

ameter of 10 cm and width of 4 mm, this particular design

would correspond to an inductance of approximately 15 mH

[14],[25].

B. Electric Propulsion Motor. In this system, surface permanent magnet synchronous mo-

tors (SPMSM) are used as the electric propulsion due to the

high power density and high efficiency [26]. The power capac-

ity, number of pair poles, and nominal speed of each propulsion

motor is 1.86 MW, 4, and 4000 rpm, respectively.

The principle of controlling the motors is based on the Field

Oriented Control (FOC) strategy [27], as shown in Fig. 2. To

implement the FOC strategy, the control unit translates the sta-

tor variables (currents) into a d-q frame coordination based on

the rotor position as well as to compare the values with the ref-

erence values (ωref, iqref and idref), and updates the PI controllers.

The inverter gate signals are updated after the back transfor-

mation of the new voltage references into the stator frame co-

ordination and compared with the modulating signals. In order

to achieve the Maximum Torque Per Ampere (MTPA) strategy,

idref is set to zero for the whole time [28] and the gains of all PI

blocks have been fine-tuned by control theory analysis together

with trial and error adjustments. In Fig. 2, S1-S6 are insulated-

gate bipolar transistors (IGBTs) with integrated, anti-parallel

diodes.

Fig. 2: Field Oriented Control (FOC) for an electric propulsion motor.

III. SMES-FCL CONTROL METHOD

The multifunctional superconducting coils are designed to

work in two different operation modes based on the fault posi-

tion. If the fault occurs on the propulsion side, as is the case in

Fault #1, part of the superconducting coils work as a FCL to

limit the fault current and the rest of the coils are isolated to

protect them from the overcurrent. However, if the fault occurs

on the generation side, as is the case in Fault #2 in Fig.1, the

superconducting coil works as a SMES to maintain the speed of

the propulsion system and the voltage at the DC-link.

The SMES-FCL device is programmed based on the current and

the voltage measurements on the DC bus. If the fault occurs at

the generation side as in Fault #2, both the current and voltage

on the DC bus drop; as a result, the SMES-FCL works as a

SMES in discharge mode, as shown in Fig. 3 (a). During the

normal operation, the switches, S1, S4 and S5 are closed, S2,

S3 and S6 are open. When the SMES-FCL works in SMES op-

eration mode, the switches I1 and I2 control the discharge rate

of the SMES based on the control algorithms which allow the

two switches to discharge the appropriate amount of current to

maintain the DC-link voltage at the acceptable level and the

motor speed at the reference speed.

If the fault occurs on the propulsion side as in Fault #1, the cur-

rent increases dramatically and the DC bus voltage drops. In

this case, the SMES-FCL works as a FCL to limit the fault cur-

rent and isolate the rest of the coils for protection, as shown in

Fig. 3 (b), whereby two pancakes are used as a FCL, as shown

by the red arrows, and the rest of the coils are isolated for pro-

tection, as shown by the green arrows. For the FCL mode, the

switching sequence is: I1 is off and I2 is on to put the coils in

the standby mode. Then, S2, S3, and S6 are closed to isolate the

SMES coils for protection. Finally, S1, S4 and S5 are opened to

force the current to go through the two pancakes which is the

FCL part to limit the fault current. In Fig. 3, S1-S6 are unidi-

rectional, reverse blocking IGBTs. More details pertaining to

controlling I1 and I2 in the SMES mode can be found in [20].

Fig. 3 (c) shows the SMES charging mode of the SMES-FCL,

and Fig. 3 (d) shows the SMES standby mode.

ABC

dq

ABC

dqPID

-

+PID PID

+

-0

+

-Pulse

Generator

S1

S4

S3

S6

S5

S2

S4S1

S5S2

S6S3

DC-Link

M

IV. SIMULATION RESULTS & DISCUSSION

The power system architecture of the EA shown in Fig. 1 is

modelled in the SimPower™ Simcape™ systems environments

to test the performance of the SMES-FCL devices under differ-

ent fault scenarios. Based on the control topology, Fault #1

made the SMES-FCL device work in the FCL mode, while

Fault #2 made the SMES-FCL device work in the SMES mode.

To show different types of DC faults in the DC microgrid of the

EA, a pole-to-pole and pole-to-ground faults are applied at the

location of Fault #1, while Fault #2 is a pole-to-ground fault

[29]. The aircraft grounding method employed in this paper is

based on a current return network (CRN) formed by the tradi-

tional metallic aircraft structure, with additional cables where

required, thus ensuring a low electrical impedance (max. 0.1 to

0.2 ohms) [7], [30].

1) FCL mode (Fault #1): When the system is subjected to a pole-to-pole fault current

at the propulsion side, the SMES-FCL works in the FCL mode

to limit the high current, as shown in Fig. 3(b). The fault lasts

for 100 ms, from 5.0 s to 5.1 s. The current in the main feeder

is shown in Fig. 4(a) in three cases: With and without the

SMES-FCL device and with the separate SMES and FCL de-

vices. Without the SMES-FCL, the current increases dramati-

cally, whereas with the combined SMES-FCL, the FCL is able

to limit the current to almost twice the rated current in few mil-

liseconds. Fig. 4(b) shows the voltage at the generation side;

without the SMES-FCL, the voltage drops to almost 0.23 pu of

the nominal voltage, whereas with the SMES-FCL, the voltage

is maintained above 0.86 pu. The generator speed is maintained

above 0.98 pu of the nominal speed with the SMES-FCL; how-

ever, the speed drops to almost 0.94 pu of the nominal speed

without the SMES-FCL device, as shown in Fig. 4 (c). It is no-

ticeable that the separate FCL device works slightly faster than

the combined SMES-FCL, but that is due to the latter using.

switches. For all results in this section, the circuit breakers do

not trip, showcasing the performance of the SMES-FCL device.

(a)

0

2000

4000

6000

8000

4.8 4.9 5 5.1 5.2 5.3

Curr

ent (

A)

Time (s)

Without SMES_FCL

Combined SMES_FCL

Separate SMES_FCL

Vbus

S2

S665 Pancakes

S1 NC

Generation

SidePropulsion

Side

2 PancakesNC

S3

S4

S5

NONO

NC

NO

I1

I2

D1

D2

Vbus

S2S665 Pancakes

S1 NC

Generation

Side

Propulsion

Side

NC

S3

S4

S5

NONO

NC

NO

2 Pancakes

I1 D1

I2D2

Vbus

S2S665 Pancakes

S1 NC

Generation

SidePropulsion

Side

2 Pancakes NC

S3

S4

S5

NONO NO

I1 D1

I2D2

NC

Fig. 3: SMES-FCL control topology, (a) SMES discharge mode, (b) FCL mode, (c) SMES charge mode, and (d) SMES standby mode.

(a) (b)

(c) (d)

Vbus

S2S6

65 Pancakes

S1 NCGeneration

Side

Propulsion

Side

NC

S3

S4

S5

NONO

NC

NO

2 Pancakes

I1 D1

I2D2

(b)

(c)

Fig. 4: Pole-to-pole fault on the propulsion side Fault #1 (a) Current without

SMES-FCL, with combined SMES-FCL, and with separate SMES-FCL, (b)

Voltage without SMES-FCL, with combined SMES-FCL, and with separate

SMES-FCL, (c) Generator speed without SMES-FCL, with combined SMES-

FCL, and with separate SMES-FCL

(a)

(b)

(c)

Fig. 5: Pole-to-ground fault on the propulsion side Fault #1 (a) Current with-

out SMES-FCL, with combined SMES-FCL, and with separate SMES-FCL,

(b) Voltage without SMES-FCL, with combined SMES-FCL, and with sepa-

rate SMES-FCL, (c) Generator speed without SMES-FCL, with combined

SMES-FCL, and with separate SMES-FCL.

Fig. 5 shows the effect of a pole-to-ground fault at the location

of Fault #1 on the current, the bus voltage, and the generator

speed in the same three cases explored in Fig. 4. It is clear that

these effects are larger in magnitude during a pole-to-pole fault

than in the pole-to-ground fault due to the higher potential volt-

age in the pole-to-pole fault.

2) SMES mode (Fault #2):

If the fault current occurs on the generation side, the SMES-

FCL works in the SMES mode to maintain the voltage at the

required level and maintain the propulsion system speed at the

desired speed. When both the voltage and current drop at the

DC-link, the SMES-FCL works as a SMES by discharging cur-

rent to supply the propulsion system. The system was subjected

to a pole-to-ground fault at the position of Fault #2, as shown

in Fig. 1. The fault lasts for 100 ms from 5.0 s to 5.1s. The sys-

tem works in the discharged mode, as shown in Fig. 3 (a). The

SMES is able to maintain the voltage at the required level and

the motors’ speed at the reference speed. Fig. 6 (a) shows the

speed of motor MR3 in the three different cases. Fig. 6 (b)

shows the voltage at the DC-link in the three different cases.

When the voltage dropped due to the fault current, the SMES

discharged within few milliseconds as shown in in Fig. 6 (c).

Here, the combined SMES-FCL and the separate devices act in

the same way with no additional delay since the additional

switches (S1-S6) are not used in this scenario.

(a)

(b)

(c)

Fig. 6: Pole-to-ground fault on the generation side Fault #2 (a) Propulsion motor

speed of MR3 without SMES-FCL, with combined SMES-FCL, with separate

SMES-FCL, (b) Voltage without SMES-FCL, with combined SMES-FCL, and

with separate SMES-FCL, (c) SMES current with combined SMES-FCL and

with separate SMES-FCL.

V. CONCLUSIONS

This paper proposed the use of multifunction superconduct-

ing coils in the EA using a novel control technique. The pro-

posed SMES-FCL can reduce the weight as well as the cost of

EA by using the same coils in different operation modes. How-

ever, one of the potential negative side-effects of the combined

SMES-FCL is the complexity of the design. Also, the SMES

and FCL can work simultaneously when they are separate,

whereas it is not possible in the combined device. And finally,

the separate SMES and FCL can respond to faults slightly faster

than the combined SMES-FCL due to the use of switches in the

combined device.

0

2000

4000

6000

8000

4.8 4.9 5 5.1 5.2 5.3

Vo

ltag

e (V

DC

)

Time (s)

Without SMES_FCL

Combined SMES_FCL

Separate SMES_FCL

0.92

0.94

0.96

0.98

1

1.02

4.8 5.3 5.8 6.3 6.8

Gen

erat

or

Spee

d

(p.u

)

Time (s)

Without SMES_FCL

Combined SMES_FCL

Separate SMES_FCL

0

1000

2000

3000

4000

5000

4.8 4.9 5 5.1 5.2 5.3

Cu

rren

t (A

)

Time (s)

Without SMES_FCL

Combined SMES_FCL

Separate SMES-FCL

3000

4000

5000

6000

7000

4.8 4.9 5 5.1 5.2 5.3

Vo

ltag

e (V

DC

)

Time (s)

Without SMES_FCL

Combined SMES_FCL

Separate SMES_FCL

0.94

0.96

0.98

1

1.02

4.8 5.3 5.8 6.3 6.8

Ge

ne

rato

r Sp

eed

(p

.u)

Time (s)

Without SMES_FCL

Combined SMES_FCL

Separate SMES_FCL

2000

3000

4000

5000

6000

7000

4.9 5 5.1 5.2 5.3

Mot

or S

peed

(RPM

)

Time (s)

Without SMES_FCL

Combined SMES_FCL

Separate SMES_FCL

3000

4000

5000

6000

7000

4.8 4.9 5 5.1 5.2 5.3

DC

Bus

Vol

tage

(V)

Time (s)

Without SMES_FCL

Combined SMES_FCL

Separate SMES_FCL

1200

1400

1600

1800

2000

2200

2400

4.8 4.9 5 5.1 5.2 5.3

SMES

Cur

rent

(A)

Time (s)

Combined SMES_FCL

Separate SMES_FCL

The SMES-FCL device has been tested in three different fault

scenarios. In the FCL mode, the SMES-FCL device was able to

reduce the fault current from 7 times to almost twice the rated

current within a few milliseconds, maintaining the voltage at

above 0.86 pu of the nominal voltage, instead of 0.23 pu with-

out the SMES-FCL device, and maintaining the generator speed

at above 0.98 pu, instead of 0.94 pu without the SMES-FCL

device. In the SMES mode, the SMES-FCL device was able to

maintain the propulsion system speed at the required speed and

the voltage at the DC-link at the reference voltage.

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AIAA/ASME/SAE/ASEE Jt. Propuls. Conf. Exhib., no. August, pp. 1–23, 2012.

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